1. Field of the Invention
The present invention relates to a color image pickup device and a light-receiving device.
2. Related Background Art
Conventionally, as light-receiving devices, devices using p-n junctions or p-i-n junctions of compound semiconductors such as crystalline silicon, amorphous silicon and GaAs have been generally used. These light-receiving devices are two-dimensionally arranged to form a plane-type image pickup device, or one-dimensionally arranged to form a line sensor.
In the color image pickup device and line sensor, a color filter system in which color filters each allowing light of a specific wavelength range to pass through are placed is generally used for color separation. That is, in the color image pickup device, color filters 701 are placed on the top of CCD or CMOS sensors 702 as shown in FIGS. 22A and 22B. Examples of color filter systems include a color filter system in which color filters of three primary colors: red (hereinafter abbreviated as R), green (hereinafter abbreviated as G) and blue (hereinafter abbreviated as B) are placed, and a color filter system involving color separation into cyanogen, magenta, yellow and green as complementary color filters.
Also, systems for high image quality include a multiple plate system in which a color image is separated by a color separation prism, and three to four image pickup devices are used. For example, incident light is color-separated by using a prism, and thereafter CCDs are placed-for three colors of R, G and B. In addition, a four-plates system employing two CCDs for G for improving the resolution is known.
However, the color filter system has the following problems: (1) the light amount decreases because a part of light is absorbed by the color filter; and (2) a false color is produced because color separation is done by detecting different colors at different locations. Also, an optical lowpass filter is required for preventing the problem of false color, and a loss of light occurs also in this case.
In the multiple plate system, on the other hand, a high accuracy prism or color separating film (dichroic mirror) is required, and a high level of registration technique is required, thus raising problems such that the cost is increased, the equipment is scaled up and a loss of light occurs in an optical device such as a prism.
For solving the problem of false color, there is a stacked-type image sensor structure. That is, if light-receiving devices sensitive to different colors can be stacked, color separation can be performed at the same (in-plane) location, thus making it possible to prevent the problem of false color.
One example of the stacked-type image sensor is disclosed in U.S. Pat. No. 5,965,875. This structure has light-receiving parts stacked utilizing the dependency of the absorption coefficient of Si on the wavelength, in which color separation is performed in the depth direction thereof. FIG. 23 shows dependency of the absorption coefficient of Si on the wavelength, wherein as the wavelength increases, the level of absorption decreases, and light incident on the surface can enter a long distance in the depth direction. Utilizing this nature, P-N junctions that are light-receiving parts are stacked in three layers in the depth direction, in which light of a shorter wavelength is detected in a light-receiving part located closer to the surface, and light of a longer wavelength is detected in a light-receiving part existing at a deeper location.
This image pickup device is effective for the false color, but has a disadvantage that as far as using light penetration depth dependence on the wavelength in Si, the spectrum range detected in each stacked light-receiving part is broad, and thus color separation is inadequately performed. Specifically, light of a long wavelength (e.g. red) is adsorbed even in a light-receiving part (e.g. blue) detecting light of a short wavelength, and conversely light of a short wavelength (e.g. blue) is absorbed even in a light-receiving part (e.g. red) detecting light of a long wavelength. This raises a problem such that the amount of light substantially converted into a signal decreases to reduce the sensitivity. Also, color separation can be controlled to some extent by designing the depth of the P-N junction of each light receiving part, but there is a tradeoff relationship between the sensitivity and the color separation, and the design is severely limited, for example the improvement in color separation results in a reduction in sensitivity.
It is especially difficult to achieve compatibility between the sensitivity and the color separation for green of high visual sensitivity. For example, in an ordinary visible color image sensor, red, green and blue have peak sensitivities at about 450 nm, about 550 nm and about 650 to 700 nm, respectively, but this device is poor in color purity because the sensitivity peak of green is at 500 nm, which is slightly shifted toward the short wavelength region, and the sensitivity of red is shifted toward the long wavelength region.
In addition, due to the three-layer stacked structure, structures such as a wiring are complicated, fabrication work becomes difficult, and thus the cost is increased.